U.S. patent application number 09/964839 was filed with the patent office on 2002-06-06 for optical filter.
Invention is credited to Shigehara, Masakazu, Suganuma, Hiroshi, Takushima, Michiko.
Application Number | 20020067884 09/964839 |
Document ID | / |
Family ID | 18837782 |
Filed Date | 2002-06-06 |
United States Patent
Application |
20020067884 |
Kind Code |
A1 |
Takushima, Michiko ; et
al. |
June 6, 2002 |
Optical filter
Abstract
An optical filter has a first long-period grating and a second
long-period grating formed in a unitary optical fiber. A difference
is not less than 100 nm between a wavelength at which optical
coupling is maximum between core-mode light and cladding-mode light
of a predetermined mode number in the first long-period grating and
a wavelength at which optical coupling is maximum between core-mode
light and cladding-mode light of the same mode number as the
cladding-mode light of the predetermined mode number, in the second
long-period grating. This optical filter can be constructed in
compact size and can readily implement a desired transmission
characteristic.
Inventors: |
Takushima, Michiko;
(Yokohama-shi, JP) ; Shigehara, Masakazu;
(Yokohama-shi, JP) ; Suganuma, Hiroshi;
(Yokohama-shi, JP) |
Correspondence
Address: |
MCDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Family ID: |
18837782 |
Appl. No.: |
09/964839 |
Filed: |
September 28, 2001 |
Current U.S.
Class: |
385/27 ;
385/37 |
Current CPC
Class: |
G02B 6/0208 20130101;
G02B 6/29319 20130101; G02B 6/02095 20130101 |
Class at
Publication: |
385/27 ;
385/37 |
International
Class: |
G02B 006/26; G02B
006/34 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 1, 2000 |
JP |
P2000-367344 |
Claims
What is claimed is:
1. An optical filter for attenuating light of a predetermined
wavelength in a communication wavelength band, said optical filter
comprising: a unitary optical waveguide having a core region and a
cladding region; and a plurality of long-period gratings formed
each in said optical waveguide, wherein, in first and second
long-period gratings selected from said plurality of long-period
gratings, a difference is not less than 100 nm between a wavelength
at which optical coupling is maximum between core-mode light and
cladding-mode light of a predetermined mode number in said first
long-period grating and a wavelength at which optical coupling is
maximum between core-mode light and cladding-mode light of the same
mode number as said cladding-mode light of said predetermined mode
number, in said second long-period grating.
2. The optical filter according to claim 1, wherein said difference
is not less than 200 nm.
3. The optical filter according to claim 1, wherein at least one of
said first and second long-period gratings has a phase rotating
portion.
4. The optical filter according to claim 1, wherein said
communication wavelength band includes a C-band(1525 nm to 1565 nm)
and an L-band(1565 nm to 1610 nm), wherein a wavelength at which
optical coupling is maximum in said communication wavelength band
between the core-mode light and cladding-mode light in said first
long-period grating lies in the L-band, and a wavelength at which
optical coupling is maximum in said communication wavelength band
between the core-mode light and cladding-mode light in said second
long-period grating lies in the C-band.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical filter in which
long-period gratings (LPGs) are formed in an optical waveguide.
Unlike a short-period grating reflecting light of a specific
wavelength, the long-period grating is grating converting light of
a specific wavelength among core-mode light propagating in a
confined state in the core region, into cladding-mode light and
radiating the cladding-mode light to the outside of the cladding
region, for example, as disclosed in U.S. Pat. No. 5,703,978.
[0003] 2. Related Background Art
[0004] An optical waveguide (e.g., an optical fiber) in which
refractive index modulation having the period of several hundred
.mu.m (long-period grating) is formed in an optical waveguide
region, converts light of a specific wavelength among the core-mode
light propagating in a confined state in the core region of the
optical waveguide, into cladding-mode light and radiates the
cladding-mode light to the outside of the cladding region. Namely,
the optical waveguide with the long-period grating formed therein
acts as an optical filter having wavelength selectivity. This
optical filter is characterized by nonreflective nature, as
apparent from the aforementioned loss producing mechanism.
Therefore, the optical filter is suitably applied to uses to
attenuate the core-mode light of a specific wavelength with no
reflection, and is suitably applicable, for example, as a gain
equalizer for equalizing gains of an optical amplifier in
wavelength division multiplexing (WDM) optical communications.
[0005] An optical filter with a normal long-period grating of
uniform period formed in the core region of the optical waveguide
demonstrates a transmission characteristic having the shape of the
Gaussian function in the wavelength band of 100 nm and having only
one loss peak, as shown in FIG. 9. However, the optical filter used
as the above-stated gain equalizer or the like is required to have
a complex transmission characteristic as shown in FIG. 10. In order
to meet this request, the optical filter as shown in FIG. 11 was
proposed. This optical filter is produced in the following manner.
Namely, first prepared are a plurality of optical waveguides (three
in the figure). Each of the optical waveguides has a long-period
grating and has margins at both ends of the grating. Then each of
the long-period gratings is housed in a package, the optical
waveguides are fusion-spliced between their margins, and the
spliced portions are reinforced by a reinforcement. A transmission
characteristic of this optical filter is superposition of
transmission characteristics of the respective long-period
gratings.
[0006] A conceivable reason for this is as follows. Namely, the
optical filter shown in FIG. 11 is usually coated with resin 40
around the margins for the purpose of reinforcement, as shown in
FIG. 12. Therefore, the cladding-mode light, into which the
core-mode light has been converted in the front long-period grating
LPG-a, is radiated through the resin 40 to the outside and is not
recoupled with the core-mode light in the rear long-period grating
LPG-b. For this reason, as shown in FIG. 13, the transmission
characteristic of the optical filter (indicated by L3) results in
superposition of the transmission characteristic of the long-period
grating LPG-a (indicated by L1) and the transmission characteristic
of LPG-b (indicated by L2). The transmission characteristic shown
in FIG. 13 is that of the optical filter wherein in the long-period
grating LPG-a the period of refractive index modulation is 361
.mu.m and the grating length 13 mm and wherein in the long-period
grating LPG-b the period of refractive index modulation is 356
.mu.m and the grating length 15 mm. However, the optical filter in
the structure wherein the margins are fusion-spliced as shown in
FIG. 11 is of large size and it is thus difficult to house the
entire filter in a package.
[0007] In order to make the whole filter compact enough to be
housed in a package, it can be contemplated that the optical filter
is constructed by forming a plurality of long-period gratings in
tandem in a unitary optical waveguide. In this configuration, there
is no need for provision of the margins for fusion splicing and
thus the whole of the optical filter can be made compact. However,
the transmission characteristic of the optical filter of this
structure is different from a desired one, which is superposition
of transmission characteristics of the respective long-period
gratings in the unitary optical waveguide.
[0008] A conceivable reason for this is as follows. Supposing two
long-period gratings LPG-a and LPG-b are formed in tandem in a
unitary optical waveguide to constitute an optical filter, as shown
in FIG. 14, the cladding-mode light, into which the core-mode light
has been converted in the front long-period grating LPG-a, will be
recoupled with the core-mode light in the rear long-period grating
LPG-b. For this reason, the transmission characteristic of this
optical filter becomes complex and different from superposition of
transmission characteristics of the respective long-period gratings
LPG-a and LPG-b, as shown in FIG. 15, and prediction of the
characteristic is not easy. Each of the long-period gratings LPG-a
and LPG-b herein is of structure similar to that shown in FIG. 13,
and the spacing between them is 2 mm.
[0009] In contrast with it, Document "M. Harumoto et al., "Compact
long-period grating module with multi-attenuation peaks," Electron
Lett., Vol. 36, No. 6, pp.512-514 (2000)" describes the optical
filter that can be formed in compact size and that can readily
implement a desired transmission characteristic. The optical filter
described in this Document is one wherein two long-period gratings
LPG-a and LPG-b5 are formed in tandem in a unitary optical
waveguide, as shown in FIG. 16. In this optical filter, a mode
number of cladding-mode light to be coupled with core-mode light in
the long-period grating LPG-a is different from that in LPG-b5 in
the waveguide band used in communications.
[0010] For example, suppose in the front long-period grating LPG-a
the period of refractive index modulation is 361 .mu.m and the
grating length 13 mm, in the rear long-period grating LPG-b5 the
period of refractive index modulation is 415 .mu.m and the grating
length 14 mm, and the spacing between them is 2 mm. In this
configuration, the mode numbers of the cladding-mode light to be
coupled with the core-mode light in the communication wavelength
band (1525 nm to 1565 nm; so-called C-band) are six in the front
long-period grating LPG-a and five in the rear long-period grating
LPG-b5. Therefore, the sixth cladding-mode light, into which the
core-mode light has been converted in the front long-period grating
LPG-a, is rarely recoupled with the core-mode light in the rear
long-period grating LPG-b5. Therefore, as shown in FIG. 17, the
transmission characteristic of this optical filter (indicated by
L1) becomes nearly equal to superposition of transmission
characteristics of the two long-period gratings LPG-a and LPG-b5
(indicated by L2).
SUMMARY OF THE INVENTION
[0011] The inventors conducted research on the above prior art and
found the following problem. Specifically, FIG. 17 also shows the
transmission characteristic of the configuration with the spliced
portion between the two long-period gratings LPG-a and LPG-b5
(indicated by L2) and a difference between them (indicated by L3),
in addition to the transmission characteristic of the configuration
with the two long-period gratings LPG-a and LPG-b5 formed in tandem
in the unitary optical waveguide (indicated by L1). As seen from
this figure, though the mode numbers of the cladding-mode light to
be coupled with the core-mode light in the C-band are made
different from each other between the two long-period gratings
LPG-a and LPG-b5, the transmission characteristic of the
configuration with the two gratings formed in tandem in the unitary
optical waveguide is different by about -0.25 dB to +0.32 dB in the
C-band from the transmission characteristic of the configuration
with the spliced portion between the two gratings.
[0012] As described above, the transmission characteristic of the
optical filter described in aforementioned Document is also
different from the superposition of the transmission
characteristics of the respective long-period gratings LPG-a and
LPG-b5, and prediction of the characteristic is not easy. For
example, in the case of a long-haul optical communication system
the transmission distance of which is even several thousand km, the
difference can adversely affect the operation of the system and
result in degradation of communication characteristics.
[0013] The present invention has been accomplished in order to
solve the above problem and an object of the invention is to
provide an optical filter that can be constructed in compact size
and that can readily implement a desired transmission
characteristic.
[0014] An optical filter according to the present invention is an
optical filter for attenuating light of a predetermined wavelength
in a communication wavelength band. This optical filter comprises a
unitary optical waveguide having a core region and a cladding
region, and a plurality of long-period gratings formed in the
optical waveguide. In first and second long-period gratings
selected from the plurality of long-period gratings, a difference
is not less than 100 nm between a wavelength at which optical
coupling becomes maximum between core-mode light and cladding-mode
light of a predetermined mode number in the first long-period
grating and a wavelength at which optical coupling becomes maximum
between core-mode light and cladding-mode light of the same mode
number as the cladding-mode light of said predetermined mode
number, in the second long-period grating. More preferably, the
above difference is not less than 200 nm. This allows the optical
filter to be constructed in compact size and to readily implement a
desired transmission characteristic.
[0015] In the optical filter according to the present invention, at
least one of the first and second long-period gratings can have a
phase rotating portion. This configuration makes it easier to
implement a desired transmission characteristic.
[0016] In the optical filter according to the present invention,
the communication wavelength band includes a C-band (1525 nm to
1565 nm) and an L-band (1565 nm to 1610 nm), a wavelength at which
optical coupling is maximum in the communication wavelength band
between the core-mode light and cladding-mode light in the first
long-period grating lies in the L-band, and a wavelength at which
optical coupling becomes maximum in the communication wavelength
band between the core-mode light and cladding-mode light in the
second long-period grating lies in the C-band. This optical filter
can be suitably applied to a wavelength division multiplexing
optical communication system utilizing both the C-band and the
L-band.
[0017] The present invention can be further fully understood from
the detailed description and accompanying drawings which will
follow. It is to be considered that these are presented simply for
illustration of the invention and do not limit the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a structural view of an optical filter according
to an embodiment of the present invention.
[0019] FIGS. 2A and 2B are graphs showing transmission
characteristics of the second long-period grating and the optical
filter in a first example.
[0020] FIGS. 3A and 3B are graphs showing transmission
characteristics of the second long-period grating and the optical
filter in a second example.
[0021] FIGS. 4A, 4B, and 4C are graphs showing transmission
characteristics of the first long-period grating, the second
long-period grating, and the optical filter in a third example.
[0022] FIG. 5 is an explanatory diagram to illustrate a
phase-shifted long-period grating.
[0023] FIG. 6 is a diagram showing a transmission characteristic of
the phase-shifted long-period grating.
[0024] FIG. 7 is a graph showing a transmission characteristic of
the optical filter consisting of the phase-shifted long-period
grating and the normal long-period grating in a fourth example.
[0025] FIG. 8 is a graph showing a transmission characteristic of
the optical filter consisting of the phase-shifted long-period
grating and the normal long-period grating in a fifth example.
[0026] FIG. 9 is a graph showing the transmission characteristic of
the optical filter in which the normal long-period grating of
uniform period is formed in the core region of the optical
waveguide.
[0027] FIG. 10 is a graph showing the transmission characteristic
required of the optical filter used as a gain equalizer.
[0028] FIG. 11 is a structural view of the optical filter in which
a plurality of optical waveguides each having a long-period grating
are fusion-spliced between their margins.
[0029] FIG. 12 is a structural view of the optical filter in which
two optical waveguides each having a long-period grating are
fusion-spliced between their margins(The fusion-spliced portion is
not shown).
[0030] FIG. 13 is a graph showing the transmission characteristic
of the optical filter in which two optical waveguides each having a
long-period grating are fusion-spliced between their margins.
[0031] FIG. 14 is a structural view of the optical filter in which
two long-period gratings are formed in a unitary optical
waveguide.
[0032] FIG. 15 is a graph showing the transmission characteristic
of the optical filter in which two long-period gratings are formed
in a unitary optical waveguide.
[0033] FIG. 16 is a structural view of the optical filter in which
two long-period gratings are formed in a unitary optical
waveguide.
[0034] FIG. 17 is a graph showing the transmission characteristic
of the optical filter in which two long-period gratings are formed
in a unitary optical waveguide.
[0035] FIGS. 18A and 18B are graphs showing transmission
characteristics resulting from coupling between core-mode light and
cladding-mode light of each mode number in a single body of
long-period grating LPG-a.
[0036] FIG. 19 is a graph showing transmission characteristics due
to coupling between core-mode light and cladding-mode light of each
mode number in a single body of long-period grating LPG-b5.
[0037] FIG. 20 is an explanatory diagram for explaining mode
coupling in the optical filter in which the two long-period
gratings LPG-a and LPG-b5 are formed in a unitary optical
waveguide.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Embodiments of the present invention will be described below
in detail with reference to the accompanying drawings. Throughout
the description of the drawings the same elements will be denoted
by the same reference symbols and redundant description will be
omitted.
[0039] First, the reason for the characteristic error as shown in
FIG. 17 will be discussed referring to FIGS. 18A, 18B, FIG. 19, and
FIG. 20. FIGS. 18A and 18B are graphs showing the transmission
characteristics resulting from coupling between core-mode light and
cladding-mode light of each mode number in a single body of
long-period grating LPG-a, wherein FIG. 18B is an enlarged view of
FIG. 18A. FIG. 19 is a graph showing the transmission
characteristics resulting from coupling between core-mode light and
cladding-mode light of each mode number in a single body of
long-period grating LPG-b5. FIG. 20 is an explanatory diagram for
explaining mode coupling in an optical filter in which the two
long-period gratings LPG-a and LPG-b5 are formed in a unitary
optical waveguide. Each of the long-period gratings LPG-a and
LPG-b5 herein is of structure similar to that shown in FIGS. 16 and
17.
[0040] As seen from FIGS. 18A and 18B, the sixth cladding-mode
light is strongly coupled with the core-mode light in the
communication wavelength band (1525 nm to 1565 nm; so-called
C-band) in the single body of long-period grating LPG-a, and the
loss of the core-mode light reaches the peak value of about 2.8 dB
near the wavelength of 1560 nm because of the coupling with the
sixth cladding-mode light. As seen from FIG. 19, the fifth
cladding-mode light is strongly coupled with the core-mode light in
the C-band in the single body of long-period grating LPG-b5, and
the loss of the core-mode light reaches the peak value of about 2.3
dB near the wavelength of 1540 nm because of the coupling with the
fifth cladding-mode light.
[0041] As seen from FIG. 18B of the enlarged view, however, while
the sixth cladding-mode light is strongly coupled with the
core-mode light, the fifth cladding-mode light is weakly coupled
with the core-mode light in the C-band in the single body of
long-period grating LPG-a. The weak coupling between the core-mode
light and the fifth cladding-mode light in the long-period grating
LPG-a is due to influence of the foot portion of the peak near the
wavelength of 1460 nm at which the coupling becomes maximum between
the core-mode light and the fifth cladding-mode light. On the other
hand, while the fifth cladding-mode light is strongly coupled with
the core-mode light, the sixth cladding-mode light is also weakly
coupled with the core-mode light in the C-band in the single body
of long-period grating LPG-b5. The weak coupling between the
core-mode light and the sixth cladding-mode light in the
long-period grating LPG-b5 is due to influence of the foot portion
of the peak near the wavelength of 1700 nm at which the coupling
becomes maximum between the core-mode light and the sixth
cladding-mode light.
[0042] Accordingly, as shown in FIG. 20, the fifth cladding-mode
light appearing a little in the front long-period grating LPG-a is
recoupled with the core-mode light in the rear long-period grating
LPG-b5. The sixth cladding-mode light, into which the core-mode
light has been converted in the front long-period grating LPG-a, is
slightly recoupled with the core-mode light in the rear long-period
grating LPG-b5. For this reason, the transmission characteristic of
the optical filter as shown in FIG. 16 is a little different from
the superposition of the transmission characteristics of the
respective long-period gratings LPG-a and LPG-b5, as shown in FIG.
17.
[0043] As described above, this difference is due to the influence
of the foot portion of the peak near the wavelength of 1460 nm at
which the coupling is maximum between the core-mode light and the
fifth cladding-mode light in the long-period grating LPG-a and the
influence of the foot portion of the peak near the wavelength of
1700 nm at which the coupling is maximum between the core-mode
light and the sixth cladding-mode light in the long-period grating
LPG-b5.
[0044] It is thus speculated that the above influence can be
relaxed if the difference is large between the wavelength at which
the optical coupling is maximum between the core-mode light and the
cladding-mode light of the predetermined mode number in the first
long-period grating and the wavelength at which the optical
coupling is maximum between the core-mode light and the
cladding-mode light of the same mode number as the cladding-mode
light of the predetermined mode number, in the second long-period
grating. The present invention was accomplished on the basis of
Inventor's considerations as discussed above.
[0045] In the next place, an optical filter 1 according to an
embodiment of the invention will be described. FIG. 1 is a
structural view of the optical filter 1 according to the present
embodiment. This optical filter 1 has such a configuration that
first long-period grating 10 and second long-period grating 20 are
formed in a unitary optical fiber. In this filter 1, each of the
first long-period grating 10 and second long-period grating 20 is
formed in a silica based optical fiber 30. The optical fiber 30 has
a core region 31 doped with GeO.sub.2 and a cladding region 32.
Each of the first long-period grating 10 and second long-period
grating 20 is made by exposing the core region 31 to ultraviolet
light spatially intensity-modulated (e.g., laser light with the
wavelength of 248 nm emitted from a KrF excimer laser source). In
the optical fiber 30, a coating layer 33 covers the other portions
than the region where the first long-period grating 10 and second
long-period grating 20 are formed.
[0046] In this optical filter 1, the difference is not less than
100 nm between the wavelength at which the optical coupling is
maximum in the communication wavelength band between the core-mode
light and the cladding-mode light of the predetermined mode number
in the first long-period grating 10 and the wavelength at which the
optical coupling is maximum between the core-mode light and the
cladding-mode light of the same mode number as the cladding-mode
light of the predetermined mode number, in the second long-period
grating 20. This difference is more preferably not less than 200
nm.
[0047] Examples of the optical filter will be described below. In
the first and second examples, the first long-period grating 10 is
the above-stated LPG-a (the period 361 .mu.m and the grating length
13 mm), and the second long-period grating 20 in the first example
is different from that in the second example.
[0048] The first example will be described first. FIG. 2A is a
graph showing the transmission characteristic of the second
long-period grating 20 in the optical filter 1 of the first
example. FIG. 2B is a graph showing the transmission characteristic
of the optical filter 1 of the first example. In this example, the
second long-period grating 20 is LPG-b7 (the period 305 .mu.m and
the grating length 17 mm). This long-period grating LPG-b7 has the
transmission characteristic similar to that of the aforementioned
long-period grating LPG-b5 in the C-band.
[0049] In this long-period grating LPG-b7, as shown in FIG. 2A, a
peak wavelength of coupling between core-mode light and seventh
cladding-mode light is near 1542 nm, and a peak wavelength of
coupling between core-mode light and sixth cladding-mode light near
1420 nm. On the other hand, in the long-period grating LPG-a, as
shown in FIG. 18A, the peak wavelength of coupling between
core-mode light and sixth cladding-mode light is near 1560 nm, and
the peak wavelength of coupling between core-mode light and seventh
cladding-mode light over 1800 nm (not shown).
[0050] Namely, the difference is not less than 100 nm between the
wavelength of 1560 nm at which the optical coupling is maximum in
the C-band between the core-mode light and the sixth cladding-mode
light in the long-period grating LPG-a and the wavelength of 1420
nm at which the optical coupling is maximum between the sixth
cladding-mode light and the core-mode light in the long-period
grating LPG-b7. The difference is not less than 100 nm between the
wavelength of 1542 nm at which the optical coupling is maximum in
the C-band between the core-mode light and the seventh
cladding-mode light in the long-period grating LPG-b7 and the
wavelength of over 1800 nm at which the optical coupling is maximum
between the seventh cladding-mode light and the core-mode light in
the long-period grating LPG-a, either.
[0051] As shown in FIG. 2B, the transmission characteristic of the
optical filter with the long-period gratings LPG-a and LPG-b7
formed in tandem in the unitary optical fiber (indicated by L1)
exhibits good agreement with the transmission characteristic of the
configuration with the spliced portion between the two gratings
(indicated by L2), and the difference between them (indicated by
L3) is reduced to the range as small as about -0.24 dB to +0.15 dB
in the C-band.
[0052] In the next place, the second example will be described.
FIG. 3A is a graph showing the transmission characteristic of the
second long-period grating 20 in the optical filter 1 of the second
example. FIG. 3B is a graph showing the transmission characteristic
of the optical filter 1 of the second example. In this example, the
second long-period grating 20 is LPG-b8 (the period 262 .mu.m and
the grating length 19.5 mm). This long-period grating LPG-b8 has
the transmission characteristic similar to that of the
aforementioned long-period grating LPG-b5 in the C-band.
[0053] In this long-period grating LPG-b8, as shown in FIG. 3A, a
peak wavelength of coupling between core-mode light and eighth
cladding-mode light is near 1542 nm and a peak wavelength of
coupling between core-mode light and sixth cladding-mode light near
1330 nm. On the other hand, in the long-period grating LPG-a, as
shown in FIG. 18A, the peak wavelength of coupling between the
core-mode light and the sixth cladding-mode light is near 1560 nm,
and the peak wavelength of coupling between the core-mode light and
the eighth cladding-mode light over 1800 nm (not shown).
[0054] Namely, the difference is not less than 200 nm between the
wavelength of 1560 nm at which the optical coupling is maximum in
the C-band between the core-mode light and the sixth cladding-mode
light in the long-period grating LPG-a and the wavelength of 1330
nm at which the optical coupling is maximum between the sixth
cladding-mode light and the core-mode light in the long-period
grating LPG-b8. The difference is not less than 200 nm between the
wavelength of 1542 nm at which the optical coupling is maximum in
the C-band between the core-mode light and the eighth cladding-mode
light in the long-period grating LPG-b8 and the wavelength of over
1800 nm at which the optical coupling is maximum between the eighth
cladding-mode light and the core-mode light in the long-period
grating LPG-a, either.
[0055] As shown in FIG. 3B, the transmission characteristic of the
optical filter with the long-period gratings LPG-a and LPG-b8
formed in tandem in the unitary optical fiber (indicated by L1)
demonstrates good agreement with the transmission characteristic of
the configuration with the spliced portion between them (indicated
by L2), and the difference between them (indicated by L3) is
reduced to the range as small as about -0.15 dB to +0.14 dB in the
C-band.
[0056] The third example will be described next. FIG. 4A is a graph
showing the transmission characteristic of the first long-period
grating 10 in the optical filter 1 of the third example. FIG. 4B is
a graph showing the transmission characteristic of the second
long-period grating 20 in the optical filter 1 of the third
example. FIG. 4C is a graph showing the transmission characteristic
of the optical filter 1 of the third example.
[0057] In this example, the grating period of the first long-period
grating 10 is 383 .mu.m and the grating length 35 mm. The grating
period of the second long-period grating 20 is 263 .mu.m and the
grating length 30 mm.
[0058] In the first long-period grating 10, as shown in FIG. 4A, a
peak wavelength of coupling between core-mode light and sixth
cladding-mode light is near 1590 nm, and a peak wavelength of
coupling between core-mode light and eighth cladding-mode light
over 1800 nm (not shown). On the other hand, in the second
long-period grating 20, as shown in FIG. 4B, a peak wavelength of
coupling between core-mode light and sixth cladding-mode light is
near 1325 nm, and a peak wavelength of coupling between core-mode
light and eighth cladding-mode light near 1545 nm.
[0059] Namely, the difference is not less than 200 nm between the
wavelength of 1590 nm at which the optical coupling is maximum in
the communication wavelength band (1525 nm to 1610 nm; so-called
CL-band) between the core-mode light and the sixth cladding-mode
light in the first long-period grating 10 and the wavelength of
1325 nm at which the optical coupling is maximum between the sixth
cladding-mode light and the core-mode light in the second
long-period grating 20. The difference is not less than 200 nm
between the wavelength of over 1800 nm at which the optical
coupling is maximum between the core-mode light and the eighth
cladding-mode light in the first long-period grating 10 and the
wavelength of 1545 nm at which the optical coupling is maximum in
the CL-band between the eighth cladding-mode light and the
core-mode light in the second long-period grating 20, either.
[0060] As shown in FIG. 4C, the transmission characteristic of the
optical filter with the first long-period grating 10 and the second
long-period grating 20 formed in tandem in the unitary optical
fiber (indicated by L1) demonstrates good agreement with the
transmission characteristic of the configuration with the spliced
portion between them (indicated by L2), and the difference between
them (indicated by L3) is reduced to the range as small as about
-0.04 dB to +0.04 dB in the CL-band.
[0061] In this optical filter 1 of the third example, the
wavelength of 1590 nm at which the optical coupling is maximum
between the core-mode light and the sixth cladding-mode light in
the first long-period grating 10 lies in the L-band (1565 nm to
1610 nm), and the wavelength of 1545 nm at which the optical
coupling is maximum between the core-mode light and the eighth
cladding-mode light in the second long-period grating 20 lies in
the C-band (1525 nm to 1565 nm). Accordingly, this optical filter 1
can be suitably applied to the WDM optical communication systems
using both the C-band and L-band. Next, the fourth and fifth
examples will be described with reference to FIG. 5 to FIG. 8. In
these examples, the first long-period grating 10 is a phase-shifted
long-period grating having a phase rotating portion in the middle
of the grating. FIG. 5 is an explanatory view of the phase-shifted
long-period grating. In this phase-shifted long-period grating, two
regions having refractive index modulation of the same period of
358 .mu.m (respective lengths of which are 17.2 mm and 4.2 mm) are
formed a gap of 3 mm apart from each other and this part of the gap
of 3 mm is exposed to ultraviolet light so as to adjust the
refractive index there, thereby forming the phase rotating portion
35. FIG. 6 is a graph showing the transmission characteristic of
the phase-shifted long-period grating. A loss peak near the
wavelength of 1556 nm shown in this figure is due to coupling
between the core-mode light and the sixth cladding-mode light.
[0062] FIG. 7 is a graph showing the transmission characteristic of
the optical filter of the fourth example consisting of the
aforementioned phase-shifted long-period grating and a long-period
grating LPG-c5. FIG. 8 is a graph showing the transmission
characteristic of the optical filter of the fifth example
consisting of the aforementioned phase-shifted long-period grating
and a long-period grating LPG-c8.
[0063] The long-period grating LPG-c5 (the period 415 .mu.m and the
grating length 27 mm) and the long-period grating LPG-c8 (the
period 261 .mu.m and the grating length 37 mm) have similar
transmission characteristics in the C-band. However, in the C-band,
the long-period grating LPG-c5 has a peak of coupling between
core-mode light and fifth cladding-mode light and the long-period
grating LPG-c8 has a peak of coupling between core-mode light and
eighth cladding-mode light.
[0064] As shown in FIG. 7, the transmission characteristic of the
optical filter of the fourth example with the phase-shifted
long-period grating and the long-period grating LPG-c5 formed in
tandem in the unitary optical fiber (indicated by L1) exhibits good
agreement with the transmission characteristic of the configuration
with the spliced portion between them (indicated by L2), and the
difference between them (indicated by L3) is reduced to the range
as small as about -0.22 dB to +0.19 dB in the communication
wavelength band C.
[0065] As shown in FIG. 8, the transmission characteristic of the
optical filter of the fifth example with the phase-shifted
long-period grating and the long-period grating LPG-c8 formed in
tandem in the unitary optical fiber (indicated by L1) exhibits good
agreement with the transmission characteristic of the configuration
with the spliced portion between them (indicated by L2), and the
difference between them (indicated by L3) is reduced to the range
as small as about -0.03 dB to +0.03 dB in the C-band.
[0066] The present invention is not limited to the above
embodiments and a variety of modifications can be made. For
example, the above embodiments showed the configurations in which
the two long-period gratings were formed in tandem in the unitary
optical waveguide (optical fiber), but it can also be contemplated
that N (N.gtoreq.3) long-period gratings are formed in tandem in a
unitary optical waveguide. In this case, the present invention is
advantageously applied in such a way that, in any two long-period
gratings out of the N gratings, a difference is not less than 100
nm and more preferably not less than 200 nm between a wavelength at
which optical coupling is maximum between core-mode light and
cladding-mode light of a predetermined mode number in one
long-period grating and a wavelength at which optical coupling is
maximum between core-mode light and cladding-mode light of the same
mode number as the cladding-mode light of the predetermined mode
number, in the other long-period grating.
[0067] As detailed above, the optical filter according to the
present invention can be constructed in compact size and can
readily implement a desired transmission cha racteristic.
[0068] It is apparent that the present invention can be modified in
various ways, from the above description of the present invention.
It is to be understood that such modifications are within the gist
and scope of the present invention and improvements obvious to
those skilled in the art are intended to be embraced in the scope
of claims which follow.
* * * * *